Because su can nean "his,her,it's, their, have you learned how to how WHOSE it is yet? #1. su libro de él = now it MUST be tranalated as HIS book.

You might consider that also for #6 and #8.

Now, in the 3rd set:
#2 Marí es su hermana. María es tu hermana.

#5. Ti tío le gusta manzanas. First of all remember that the verb "gustar" does NOT work like other verbs. It is used in 3rd-person singular form when one thing is peasing to HIM. In this case, the apples (plural) ARE interesting TO your uncle. Here is what it has to be: A tu tío le gustan manzanas.

#8. Sus padres mucho trabajar. Now you have no conjugated verb here. The sentence must be: Sus padres trabajan mucho.